By 2008 more than half of the world’s population (3.3 billion people) will live in cities – an increase from 30% in 1950 to 47% in 2000 – and will probably reach 60% in 2030. In the developing countries of Asia and Africa urbanization is proceeding rapidly. Megacities, as the largest entities of worldwide urbanization, are defined as cities with more than five million inhabitants. Scientists estimate that by 2015 the world may contain as many as 60 megacities, together housing more than 600 million people.

Megacities are not just very big. Their scale creates new dynamics, new complexity and new simultaneity of events and processes – physical, social and economic. They host intense and complex interactions between different demographic, social, political, economic and ecological processes. Megacities undergoing economic boom times often generate considerable opportunities, as well as strong pressures for change accompanied by environmental degradation. In the developing world, megacities grow faster than their infrastructure. This uncontrolled urban sprawl can foster great traffic volumes, high concentrations of industrial production, ecological overload, unregulated and disparate land and property markets, insufficient housing development and, in some cases, such extremes of poverty and wealth living side by side that social unrest may follow.

Such scale and dynamism, coupled with complex interacting processes and the sheer concentration of human capital, make megacities incubators of huge growth and innovation. They are the focal points of globalization as well as the driving forces for development; they harbor a wide spectrum of human skill and potential, creativity, social interaction and cultural diversity.

Megacities are also focuses of global risk. They are increasingly vulnerable systems because they often harbor pronounced poverty, social inequality and environmental degradation, all of which are linked together by a complex system supplying goods and services. People from different socio-economic groups and corresponding political allegiances may become segregated geographically, creating disparities and conflict. Population density increases vulnerability to natural and manmade hazards. Thus, megacities, exposed to the global environmental, socio-economic and political changes to which they contribute, are both victims and producers of risk.

For many megacities, inadequate representative governance inhibits spatial planning, building control, the delivery of services (such as water supply, sewage disposal and energy distribution), and the establishment of general order (including security and disaster prevention). Existing administrations and their organizational structures may have been outgrown by the rapidly expanding city and may simply be unable to cope with the huge scale of their new responsibilities. In addition, of course, informal processes and activities can take on an important role in the development of megacities.

Among the most important research questions are those bearing upon how we can improve the quality of life for millions of people, which for many residents is commonly low, rich and poor alike. Air, water and soil pollution, water and energy supply shortages, traffic congestion, environmental health problems, limited green spaces, poverty and malnutrition, social security and public safety problems place many burdens and restrictions on people. In the megacities of the developing world, city planning needs to adapt to diverse socio-cultural circumstances by including the often widespread and dynamic informal activities that enrich such communities. Innovative management solutions for new visions of “quality of life” have to be developed. The identification of “hot spots” of urban problems such as insufficient water supply, poor health care or inadequate public safety, is important in planning strategy.

Megacities, moreover, require human and natural resources for energy, industry, construction, infrastructure and maintenance. Their sprawl commonly encroaches on areas with difficult ground conditions, subject to geohazards, such as flooding or landslides. This makes both initial development and long-term maintenance more expensive.

The effects of global environmental and socio-economic change can magnify the risks and impair quality of life for many people. Increasing population density can give rise to increased risks to people and property in the face of environmental and man-made hazards. Vigorous economic activity generates individual and societal prosperity and material wellbeing (although often at the price of greater personal, social and environmental stress). To be sustainable, authorities must control the fabric and land use patterns within a megacity to minimize adverse environmental effects. This requires integrity in resource, logistics (traffic) and waste management, with cost-effective recovery of value, recycling of wastes and materials, and, as far as practicable, reduction of risks to health.

Surface development requires sound foundation conditions. Skyscrapers, elevated roads and railways, communications, electrical and energy corridors and many places where people live, shop, relax and work are built on and above this surface. Concentrated compact developments may depress quality of life, and areas may become more vulnerable to natural and man-made hazards. Some infrastructure and development related to environmentally unfriendly or other undesirable activities at the surface could be placed underground, thereby significantly improving the quality of living space at ground level.

Geographers and Earth scientists have long contributed to the understanding and management of complex megacities through their knowledge of Earth processes as well as their ability to find and exploit natural resources. They have a unique understanding of how socio-economic and natural systems operate at scales ranging from global to local and they have a role to play in the sustainable management of megacities. Their input can ensure deeper understanding of the complex socio-economic processes, wise management of human and economic resources and reduction of risk from natural, man-made and man-enhanced hazards.